This study has two stages. Stage 1 will last a minimum of 4 weeks; Stage 2 will last a minimum of 8 weeks. In Stage 1, patients will be randomly assigned to one of two arms. Arm A will receive the fixed-dose tablet twice daily for 2 weeks, then switch to the individual liquid formulations twice daily for 2 weeks. Arm B will receive the individual liquid formulations twice daily for 2 weeks, then switch to the fixed-dose tablet twice daily for 2 weeks. To encourage medication adherence, study staff will make home visits and phone calls to each patient's parent or guardian the first week of each treatment regimen. Medical history, a physical exam, and urine collection will occur on Days 11 and 25. Also on Days 11 and 25, patients will be admitted to the hospital to ensure 100% medication adherence and to provide blood for PK studies.